This invention relates to ducts, and in particular, to helically wound flexible ducts having a stiffening element.
Helically wound flexible ducts are used in many industrial and commercial applications to transport gases, especially in the field of HVAC systems. Typically, these ducts are constructed by helically winding a stiffening element with a flexible strip of material to form a helically-wound duct. In some industrial applications, the ducts are placed in environments which require gases to be transported through confined spaces and around corners. These ducts must be sufficiently flexible to navigate through the confined spaces, while still providing adequate structural strength to support the transport of gases. In some commercial applications, such as an upright vacuum cleaner, the ducts must be capable of being axially compressed so as to fit in small storage compartments. In use, the ducts must be capable of being axially expanded to vacuum items that cannot be reached by the main vacuuming head, such as stairs, curtains etc., while providing impact resistance and radial strength under vacuum pressure.
These concerns are addressed to some extent by a duct comprised of a helically wound stiffening element having a circular cross-section. However, these ducts are typically unable to provide a combination of sufficient radial strength and compressibility for most industrial and commercial applications. For instance, a duct comprising of a stiffening element with a large circular cross-section has considerable radial strength but limited axial compressibility and flexibility. Consequently, the poor compressibility of these ducts would necessitate the use of large shipping containers and would make handling and installation in confined spaces difficult. Conversely, a duct with a stiffening element having a small circular cross-section would provide significant axial compressibility and flexibility but inadequate radial strength. When used under vacuum pressures, these ducts are prone to radial collapse.
Accordingly, there is a need for a duct having a stiffening element with a profiled cross-section that is capable of being axially compressed to fit in a confined space while still providing adequate radial strength to support the transport of fluids and withstand impact stresses.
The present invention is directed to a duct having at least one helically wound strip of flexible material and at least one stiffening element. The helically wound strip has a leading edge and a trailing edge which overlap one another on adjacent convolutions of the strip and are bonded together to form a helical seam. The stiffening element extends helically along the helical seam. The stiffening element has an axial length and radial height and the ratio of the radial height to axial length is greater than 1.0.
In a preferred embodiment, the ratio of the radial height to axial length is greater than 2.0. The ratio of the radial height to axial length may also be greater than 3.0. The stiffening element may be substantially planar. The stiffening element may also be formed from a metallic or non-metallic material.